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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="EXTEND-EXTENSIONS"
>35.15. Packaging Related Objects into an Extension</A
></H1
><P
>    A useful extension to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> typically includes
    multiple SQL objects; for example, a new data type will require new
    functions, new operators, and probably new index operator classes.
    It is helpful to collect all these objects into a single package
    to simplify database management.  <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> calls
    such a package an <I
CLASS="FIRSTTERM"
>extension</I
>.  To define an extension,
    you need at least a <I
CLASS="FIRSTTERM"
>script file</I
> that contains the
    <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> commands to create the extension's objects, and a
    <I
CLASS="FIRSTTERM"
>control file</I
> that specifies a few basic properties
    of the extension itself.  If the extension includes C code, there
    will typically also be a shared library file into which the C code
    has been built.  Once you have these files, a simple
    <A
HREF="sql-createextension.html"
>CREATE EXTENSION</A
> command loads the objects into
    your database.
   </P
><P
>    The main advantage of using an extension, rather than just running the
    <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script to load a bunch of <SPAN
CLASS="QUOTE"
>"loose"</SPAN
> objects
    into your database, is that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will then
    understand that the objects of the extension go together.  You can
    drop all the objects with a single <A
HREF="sql-dropextension.html"
>DROP EXTENSION</A
>
    command (no need to maintain a separate <SPAN
CLASS="QUOTE"
>"uninstall"</SPAN
> script).
    Even more useful, <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> knows that it should not
    dump the individual member objects of the extension &mdash; it will
    just include a <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> command in dumps, instead.
    This vastly simplifies migration to a new version of the extension
    that might contain more or different objects than the old version.
    Note however that you must have the extension's control, script, and
    other files available when loading such a dump into a new database.
   </P
><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will not let you drop an individual object
    contained in an extension, except by dropping the whole extension.
    Also, while you can change the definition of an extension member object
    (for example, via <TT
CLASS="COMMAND"
>CREATE OR REPLACE FUNCTION</TT
> for a
    function), bear in mind that the modified definition will not be dumped
    by <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>.  Such a change is usually only sensible if
    you concurrently make the same change in the extension's script file.
    (But there are special provisions for tables containing configuration
    data; see below.)
   </P
><P
>    The extension mechanism also has provisions for packaging modification
    scripts that adjust the definitions of the SQL objects contained in an
    extension.  For example, if version 1.1 of an extension adds one function
    and changes the body of another function compared to 1.0, the extension
    author can provide an <I
CLASS="FIRSTTERM"
>update script</I
> that makes just those
    two changes.  The <TT
CLASS="COMMAND"
>ALTER EXTENSION UPDATE</TT
> command can then
    be used to apply these changes and track which version of the extension
    is actually installed in a given database.
   </P
><P
>    The kinds of SQL objects that can be members of an extension are shown in
    the description of <A
HREF="sql-alterextension.html"
>ALTER EXTENSION</A
>.  Notably, objects
    that are database-cluster-wide, such as databases, roles, and tablespaces,
    cannot be extension members since an extension is only known within one
    database.  (Although an extension script is not prohibited from creating
    such objects, if it does so they will not be tracked as part of the
    extension.)  Also notice that while a table can be a member of an
    extension, its subsidiary objects such as indexes are not directly
    considered members of the extension.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN51807"
>35.15.1. Extension Files</A
></H2
><P
>     The <A
HREF="sql-createextension.html"
>CREATE EXTENSION</A
> command relies on a control
     file for each extension, which must be named the same as the extension
     with a suffix of <TT
CLASS="LITERAL"
>.control</TT
>, and must be placed in the
     installation's <TT
CLASS="LITERAL"
>SHAREDIR/extension</TT
> directory.  There
     must also be at least one <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script file, which follows the
     naming pattern
     <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>version</I
></TT
>.sql</TT
>
     (for example, <TT
CLASS="LITERAL"
>foo--1.0.sql</TT
> for version <TT
CLASS="LITERAL"
>1.0</TT
> of
     extension <TT
CLASS="LITERAL"
>foo</TT
>).  By default, the script file(s) are also
     placed in the <TT
CLASS="LITERAL"
>SHAREDIR/extension</TT
> directory; but the
     control file can specify a different directory for the script file(s).
    </P
><P
>     The file format for an extension control file is the same as for the
     <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file, namely a list of
     <TT
CLASS="REPLACEABLE"
><I
>parameter_name</I
></TT
> <TT
CLASS="LITERAL"
>=</TT
> <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
>
     assignments, one per line.  Blank lines and comments introduced by
     <TT
CLASS="LITERAL"
>#</TT
> are allowed.  Be sure to quote any value that is not
     a single word or number.
    </P
><P
>     A control file can set the following parameters:
    </P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="VARNAME"
>directory</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        The directory containing the extension's <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script
        file(s).  Unless an absolute path is given, the name is relative to
        the installation's <TT
CLASS="LITERAL"
>SHAREDIR</TT
> directory.  The
        default behavior is equivalent to specifying
        <TT
CLASS="LITERAL"
>directory = 'extension'</TT
>.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>default_version</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        The default version of the extension (the one that will be installed
        if no version is specified in <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>).  Although
        this can be omitted, that will result in <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>
        failing if no <TT
CLASS="LITERAL"
>VERSION</TT
> option appears, so you generally
        don't want to do that.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>comment</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        A comment (any string) about the extension.  Alternatively,
        the comment can be set by means of the <A
HREF="sql-comment.html"
>COMMENT</A
>
        command in the script file.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>encoding</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        The character set encoding used by the script file(s).  This should
        be specified if the script files contain any non-ASCII characters.
        Otherwise the files will be assumed to be in the database encoding.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>module_pathname</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        The value of this parameter will be substituted for each occurrence
        of <TT
CLASS="LITERAL"
>MODULE_PATHNAME</TT
> in the script file(s).  If it is not
        set, no substitution is made.  Typically, this is set to
        <TT
CLASS="LITERAL"
>$libdir/<TT
CLASS="REPLACEABLE"
><I
>shared_library_name</I
></TT
></TT
> and
        then <TT
CLASS="LITERAL"
>MODULE_PATHNAME</TT
> is used in <TT
CLASS="COMMAND"
>CREATE
        FUNCTION</TT
> commands for C-language functions, so that the script
        files do not need to hard-wire the name of the shared library.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>requires</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        A list of names of extensions that this extension depends on,
        for example <TT
CLASS="LITERAL"
>requires = 'foo, bar'</TT
>.  Those
        extensions must be installed before this one can be installed.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>superuser</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        If this parameter is <TT
CLASS="LITERAL"
>true</TT
> (which is the default),
        only superusers can create the extension or update it to a new
        version.  If it is set to <TT
CLASS="LITERAL"
>false</TT
>, just the privileges
        required to execute the commands in the installation or update script
        are required.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>relocatable</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        An extension is <I
CLASS="FIRSTTERM"
>relocatable</I
> if it is possible to move
        its contained objects into a different schema after initial creation
        of the extension.  The default is <TT
CLASS="LITERAL"
>false</TT
>, i.e. the
        extension is not relocatable.
        See below for more information.
       </P
></DD
><DT
><TT
CLASS="VARNAME"
>schema</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
>        This parameter can only be set for non-relocatable extensions.
        It forces the extension to be loaded into exactly the named schema
        and not any other.  See below for more information.
       </P
></DD
></DL
></DIV
><P
>     In addition to the primary control file
     <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>.control</TT
>,
     an extension can have secondary control files named in the style
     <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>version</I
></TT
>.control</TT
>.
     If supplied, these must be located in the script file directory.
     Secondary control files follow the same format as the primary control
     file.  Any parameters set in a secondary control file override the
     primary control file when installing or updating to that version of
     the extension.  However, the parameters <TT
CLASS="VARNAME"
>directory</TT
> and
     <TT
CLASS="VARNAME"
>default_version</TT
> cannot be set in a secondary control file.
    </P
><P
>     An extension's <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script files can contain any SQL commands,
     except for transaction control commands (<TT
CLASS="COMMAND"
>BEGIN</TT
>,
     <TT
CLASS="COMMAND"
>COMMIT</TT
>, etc) and commands that cannot be executed inside a
     transaction block (such as <TT
CLASS="COMMAND"
>VACUUM</TT
>).  This is because the
     script files are implicitly executed within a transaction block.
    </P
><P
>     An extension's <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script files can also contain lines
     beginning with <TT
CLASS="LITERAL"
>\echo</TT
>, which will be ignored (treated as
     comments) by the extension mechanism.  This provision is commonly used
     to throw an error if the script file is fed to <SPAN
CLASS="APPLICATION"
>psql</SPAN
>
     rather than being loaded via <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> (see example
     script below).  Without that, users might accidentally load the
     extension's contents as <SPAN
CLASS="QUOTE"
>"loose"</SPAN
> objects rather than as an
     extension, a state of affairs that's a bit tedious to recover from.
    </P
><P
>     While the script files can contain any characters allowed by the specified
     encoding, control files should contain only plain ASCII, because there
     is no way for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> to know what encoding a
     control file is in.  In practice this is only an issue if you want to
     use non-ASCII characters in the extension's comment.  Recommended
     practice in that case is to not use the control file <TT
CLASS="VARNAME"
>comment</TT
>
     parameter, but instead use <TT
CLASS="COMMAND"
>COMMENT ON EXTENSION</TT
>
     within a script file to set the comment.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN51925"
>35.15.2. Extension Relocatability</A
></H2
><P
>     Users often wish to load the objects contained in an extension into a
     different schema than the extension's author had in mind.  There are
     three supported levels of relocatability:
    </P
><P
></P
><UL
><LI
><P
>       A fully relocatable extension can be moved into another schema
       at any time, even after it's been loaded into a database.
       This is done with the <TT
CLASS="COMMAND"
>ALTER EXTENSION SET SCHEMA</TT
>
       command, which automatically renames all the member objects into
       the new schema.  Normally, this is only possible if the extension
       contains no internal assumptions about what schema any of its
       objects are in.  Also, the extension's objects must all be in one
       schema to begin with (ignoring objects that do not belong to any
       schema, such as procedural languages).  Mark a fully relocatable
       extension by setting <TT
CLASS="LITERAL"
>relocatable = true</TT
> in its control
       file.
      </P
></LI
><LI
><P
>       An extension might be relocatable during installation but not
       afterwards.  This is typically the case if the extension's script
       file needs to reference the target schema explicitly, for example
       in setting <TT
CLASS="LITERAL"
>search_path</TT
> properties for SQL functions.
       For such an extension, set <TT
CLASS="LITERAL"
>relocatable = false</TT
> in its
       control file, and use <TT
CLASS="LITERAL"
>@extschema@</TT
> to refer to the target
       schema in the script file.  All occurrences of this string will be
       replaced by the actual target schema's name before the script is
       executed.  The user can set the target schema using the
       <TT
CLASS="LITERAL"
>SCHEMA</TT
> option of <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>.
      </P
></LI
><LI
><P
>       If the extension does not support relocation at all, set
       <TT
CLASS="LITERAL"
>relocatable = false</TT
> in its control file, and also set
       <TT
CLASS="LITERAL"
>schema</TT
> to the name of the intended target schema.  This
       will prevent use of the <TT
CLASS="LITERAL"
>SCHEMA</TT
> option of <TT
CLASS="COMMAND"
>CREATE
       EXTENSION</TT
>, unless it specifies the same schema named in the control
       file.  This choice is typically necessary if the extension contains
       internal assumptions about schema names that can't be replaced by
       uses of <TT
CLASS="LITERAL"
>@extschema@</TT
>.  The <TT
CLASS="LITERAL"
>@extschema@</TT
>
       substitution mechanism is available in this case too, although it is
       of limited use since the schema name is determined by the control file.
      </P
></LI
></UL
><P
>     In all cases, the script file will be executed with
     <A
HREF="runtime-config-client.html#GUC-SEARCH-PATH"
>search_path</A
> initially set to point to the target
     schema; that is, <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> does the equivalent of
     this:
</P><PRE
CLASS="PROGRAMLISTING"
>SET LOCAL search_path TO @extschema@;</PRE
><P>
     This allows the objects created by the script file to go into the target
     schema.  The script file can change <TT
CLASS="VARNAME"
>search_path</TT
> if it wishes,
     but that is generally undesirable.  <TT
CLASS="VARNAME"
>search_path</TT
> is restored
     to its previous setting upon completion of <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>.
    </P
><P
>     The target schema is determined by the <TT
CLASS="VARNAME"
>schema</TT
> parameter in
     the control file if that is given, otherwise by the <TT
CLASS="LITERAL"
>SCHEMA</TT
>
     option of <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> if that is given, otherwise the
     current default object creation schema (the first one in the caller's
     <TT
CLASS="VARNAME"
>search_path</TT
>).  When the control file <TT
CLASS="VARNAME"
>schema</TT
>
     parameter is used, the target schema will be created if it doesn't
     already exist, but in the other two cases it must already exist.
    </P
><P
>     If any prerequisite extensions are listed in <TT
CLASS="VARNAME"
>requires</TT
>
     in the control file, their target schemas are appended to the initial
     setting of <TT
CLASS="VARNAME"
>search_path</TT
>.  This allows their objects to be
     visible to the new extension's script file.
    </P
><P
>     Although a non-relocatable extension can contain objects spread across
     multiple schemas, it is usually desirable to place all the objects meant
     for external use into a single schema, which is considered the extension's
     target schema.  Such an arrangement works conveniently with the default
     setting of <TT
CLASS="VARNAME"
>search_path</TT
> during creation of dependent
     extensions.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN51966"
>35.15.3. Extension Configuration Tables</A
></H2
><P
>     Some extensions include configuration tables, which contain data that
     might be added or changed by the user after installation of the
     extension.  Ordinarily, if a table is part of an extension, neither
     the table's definition nor its content will be dumped by
     <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>.  But that behavior is undesirable for a
     configuration table; any data changes made by the user need to be
     included in dumps, or the extension will behave differently after a dump
     and reload.
    </P
><P
>     To solve this problem, an extension's script file can mark a table
     it has created as a configuration table, which will cause
     <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> to include the table's contents (not its
     definition) in dumps.  To do that, call the function
     <CODE
CLASS="FUNCTION"
>pg_extension_config_dump(regclass, text)</CODE
> after creating the
     table, for example
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE my_config (key text, value text);

SELECT pg_catalog.pg_extension_config_dump('my_config', '');</PRE
><P>
     Any number of tables can be marked this way.
    </P
><P
>     When the second argument of <CODE
CLASS="FUNCTION"
>pg_extension_config_dump</CODE
> is
     an empty string, the entire contents of the table are dumped by
     <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>.  This is usually only correct if the table
     is initially empty as created by the extension script.  If there is
     a mixture of initial data and user-provided data in the table,
     the second argument of <CODE
CLASS="FUNCTION"
>pg_extension_config_dump</CODE
> provides
     a <TT
CLASS="LITERAL"
>WHERE</TT
> condition that selects the data to be dumped.
     For example, you might do
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE my_config (key text, value text, standard_entry boolean);

SELECT pg_catalog.pg_extension_config_dump('my_config', 'WHERE NOT standard_entry');</PRE
><P>
     and then make sure that <TT
CLASS="STRUCTFIELD"
>standard_entry</TT
> is true only
     in the rows created by the extension's script.
    </P
><P
>     More complicated situations, such as initially-provided rows that might
     be modified by users, can be handled by creating triggers on the
     configuration table to ensure that modified rows are marked correctly.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN51982"
>35.15.4. Extension Updates</A
></H2
><P
>     One advantage of the extension mechanism is that it provides convenient
     ways to manage updates to the SQL commands that define an extension's
     objects.  This is done by associating a version name or number with
     each released version of the extension's installation script.
     In addition, if you want users to be able to update their databases
     dynamically from one version to the next, you should provide
     <I
CLASS="FIRSTTERM"
>update scripts</I
> that make the necessary changes to go from
     one version to the next.  Update scripts have names following the pattern
     <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>oldversion</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>newversion</I
></TT
>.sql</TT
>
     (for example, <TT
CLASS="LITERAL"
>foo--1.0--1.1.sql</TT
> contains the commands to modify
     version <TT
CLASS="LITERAL"
>1.0</TT
> of extension <TT
CLASS="LITERAL"
>foo</TT
> into version
     <TT
CLASS="LITERAL"
>1.1</TT
>).
    </P
><P
>     Given that a suitable update script is available, the command
     <TT
CLASS="COMMAND"
>ALTER EXTENSION UPDATE</TT
> will update an installed extension
     to the specified new version.  The update script is run in the same
     environment that <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> provides for installation
     scripts: in particular, <TT
CLASS="VARNAME"
>search_path</TT
> is set up in the same
     way, and any new objects created by the script are automatically added
     to the extension.
    </P
><P
>     If an extension has secondary control files, the control parameters
     that are used for an update script are those associated with the script's
     target (new) version.
    </P
><P
>     The update mechanism can be used to solve an important special case:
     converting a <SPAN
CLASS="QUOTE"
>"loose"</SPAN
> collection of objects into an extension.
     Before the extension mechanism was added to
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> (in 9.1), many people wrote
     extension modules that simply created assorted unpackaged objects.
     Given an existing database containing such objects, how can we convert
     the objects into a properly packaged extension?  Dropping them and then
     doing a plain <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> is one way, but it's not
     desirable if the objects have dependencies (for example, if there are
     table columns of a data type created by the extension).  The way to fix
     this situation is to create an empty extension, then use <TT
CLASS="COMMAND"
>ALTER
     EXTENSION ADD</TT
> to attach each pre-existing object to the extension,
     then finally create any new objects that are in the current extension
     version but were not in the unpackaged release.  <TT
CLASS="COMMAND"
>CREATE
     EXTENSION</TT
> supports this case with its <TT
CLASS="LITERAL"
>FROM</TT
> <TT
CLASS="REPLACEABLE"
><I
>old_version</I
></TT
> option, which causes it to not run the
     normal installation script for the target version, but instead the update
     script named
     <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>old_version</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>target_version</I
></TT
>.sql</TT
>.
     The choice of the dummy version name to use as <TT
CLASS="REPLACEABLE"
><I
>old_version</I
></TT
> is up to the extension author, though
     <TT
CLASS="LITERAL"
>unpackaged</TT
> is a common convention.  If you have multiple
     prior versions you need to be able to update into extension style, use
     multiple dummy version names to identify them.
    </P
><P
>     <TT
CLASS="COMMAND"
>ALTER EXTENSION</TT
> is able to execute sequences of update
     script files to achieve a requested update.  For example, if only
     <TT
CLASS="LITERAL"
>foo--1.0--1.1.sql</TT
> and <TT
CLASS="LITERAL"
>foo--1.1--2.0.sql</TT
> are
     available, <TT
CLASS="COMMAND"
>ALTER EXTENSION</TT
> will apply them in sequence if an
     update to version <TT
CLASS="LITERAL"
>2.0</TT
> is requested when <TT
CLASS="LITERAL"
>1.0</TT
> is
     currently installed.
    </P
><P
>     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> doesn't assume anything about the properties
     of version names: for example, it does not know whether <TT
CLASS="LITERAL"
>1.1</TT
>
     follows <TT
CLASS="LITERAL"
>1.0</TT
>.  It just matches up the available version names
     and follows the path that requires applying the fewest update scripts.
     (A version name can actually be any string that doesn't contain
     <TT
CLASS="LITERAL"
>--</TT
> or leading or trailing <TT
CLASS="LITERAL"
>-</TT
>.)
    </P
><P
>     Sometimes it is useful to provide <SPAN
CLASS="QUOTE"
>"downgrade"</SPAN
> scripts, for
     example <TT
CLASS="LITERAL"
>foo--1.1--1.0.sql</TT
> to allow reverting the changes
     associated with version <TT
CLASS="LITERAL"
>1.1</TT
>.  If you do that, be careful
     of the possibility that a downgrade script might unexpectedly
     get applied because it yields a shorter path.  The risky case is where
     there is a <SPAN
CLASS="QUOTE"
>"fast path"</SPAN
> update script that jumps ahead several
     versions as well as a downgrade script to the fast path's start point.
     It might take fewer steps to apply the downgrade and then the fast
     path than to move ahead one version at a time.  If the downgrade script
     drops any irreplaceable objects, this will yield undesirable results.
    </P
><P
>     To check for unexpected update paths, use this command:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_extension_update_paths('<TT
CLASS="REPLACEABLE"
><I
>extension_name</I
></TT
>');</PRE
><P>
     This shows each pair of distinct known version names for the specified
     extension, together with the update path sequence that would be taken to
     get from the source version to the target version, or <TT
CLASS="LITERAL"
>NULL</TT
> if
     there is no available update path.  The path is shown in textual form
     with <TT
CLASS="LITERAL"
>--</TT
> separators.  You can use
     <TT
CLASS="LITERAL"
>regexp_split_to_array(path,'--')</TT
> if you prefer an array
     format.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN52037"
>35.15.5. Extension Example</A
></H2
><P
>     Here is a complete example of an <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>-only
     extension, a two-element composite type that can store any type of value
     in its slots, which are named <SPAN
CLASS="QUOTE"
>"k"</SPAN
> and <SPAN
CLASS="QUOTE"
>"v"</SPAN
>.  Non-text
     values are automatically coerced to text for storage.
    </P
><P
>     The script file <TT
CLASS="FILENAME"
>pair--1.0.sql</TT
> looks like this:

</P><PRE
CLASS="PROGRAMLISTING"
>-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION pair" to load this file. \quit

CREATE TYPE pair AS ( k text, v text );

CREATE OR REPLACE FUNCTION pair(anyelement, text)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair';

CREATE OR REPLACE FUNCTION pair(text, anyelement)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair';

CREATE OR REPLACE FUNCTION pair(anyelement, anyelement)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair';

CREATE OR REPLACE FUNCTION pair(text, text)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair;';

CREATE OPERATOR ~&#62; (LEFTARG = text, RIGHTARG = anyelement, PROCEDURE = pair);
CREATE OPERATOR ~&#62; (LEFTARG = anyelement, RIGHTARG = text, PROCEDURE = pair);
CREATE OPERATOR ~&#62; (LEFTARG = anyelement, RIGHTARG = anyelement, PROCEDURE = pair);
CREATE OPERATOR ~&#62; (LEFTARG = text, RIGHTARG = text, PROCEDURE = pair);</PRE
><P>
    </P
><P
>     The control file <TT
CLASS="FILENAME"
>pair.control</TT
> looks like this:

</P><PRE
CLASS="PROGRAMLISTING"
># pair extension
comment = 'A key/value pair data type'
default_version = '1.0'
relocatable = true</PRE
><P>
    </P
><P
>     While you hardly need a makefile to install these two files into the
     correct directory, you could use a <TT
CLASS="FILENAME"
>Makefile</TT
> containing this:

</P><PRE
CLASS="PROGRAMLISTING"
>EXTENSION = pair
DATA = pair--1.0.sql

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)</PRE
><P>

     This makefile relies on <ACRONYM
CLASS="ACRONYM"
>PGXS</ACRONYM
>, which is described
     in <A
HREF="extend-pgxs.html"
>Section 35.16</A
>.  The command <TT
CLASS="LITERAL"
>make install</TT
>
     will install the control and script files into the correct
     directory as reported by <SPAN
CLASS="APPLICATION"
>pg_config</SPAN
>.
    </P
><P
>     Once the files are installed, use the
     <A
HREF="sql-createextension.html"
>CREATE EXTENSION</A
> command to load the objects into
     any particular database.
    </P
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